In recent years, water-absorbent resins have been widely used in the field of hygienic materials such as disposable diapers, sanitary napkins and incontinence pads.
For water-absorbent resins as described above, crosslinked products of partially neutralized polymers of acrylic acid salt are preferred because they have many advantages, including the followings: they have better water absorption performance; their raw materials such as acrylic acid has easy industrial availability, and therefore they can be produced with stable quality and low cost; and they are more resistant to decomposition and deterioration.
An absorbent article such as a disposable diaper, a sanitary napkin and an incontinence pad comprises an absorbent material usually arranged at the central part thereof to absorb and hold a body fluid excreted from the body such as urine and menstrual blood, a liquid-permeable front sheet (a top sheet) arranged on a side to make contact with the body, and a liquid-impermeable back sheet (a back sheet) arranged opposite to the side to make contact with the body. Further, the absorbent material comprises a hydrophilic fiber such as pulp and a water-absorbent resin.
In recent years, there have been increasing demands for thinner and lighter absorbent articles in view of excellent designability, convenient portability, efficient distributability and the like. Further, there have been increasing needs for so-called eco-friendly production where resource are effectively utilized to minimizing the usage of slowly growing natural materials such as trees in view of environmental conservation. Methods of producing a thinner absorbent article which are conventionally performed include, for example, a method in which the amount of a hydrophilic fiber such as crushed wood pulp, which serves to fix a water-absorbent resin in an absorbent material, is reduced while the amount of a water-absorbent resin is increased.
An absorbent material having a low ratio of a hydrophilic fiber and a large amount of water-absorbent resin is preferred for thinner in view of a reduced bulky hydrophilic fiber and a liquid holding capacity. However, in the case that a load due to deformation, pressure and the like may be applied to an absorbent material comprising a water-absorbent resin, for example, when an infant wearing a thinner absorbent article sits down, the re-wet of a to-be-absorbed liquid (the return of a liquid) may not be able to be fully prevented. Further, such an absorbent article cannot accommodate multiple urinations, resulting in giving discomfort to a wearer.
Moreover, a large amount of a water-absorbent resin becomes a soft gel-like material when it absorbs a liquid, and a load further applied to this gel may cause the so-called “gel blocking phenomenon”, resulting in significantly reduced liquid diffusibility, which in turn may slow the speed of permeation of a liquid for an absorbent material. This “gel blocking phenomenon” is explained below. When an absorbent material containing particularly highly densified water-absorbent resins absorbs liquid, a water-absorbent resin near the surface layer absorbs the liquid to further densify a soft gel around the surface layer, and so liquid permeation into the absorbent material is inhibited, preventing the internal water-absorbent resins from efficiently absorbing the liquid.
Accordingly, the followings have been so far proposed as means for preventing problems which may occur when a hydrophilic fiber is reduced and a large amount of a water-absorbent resin is used: for example, a method in which a hydrogel absorptive polymer is used having a specific saline-flow inductivity, under-pressure performance and the like (see Patent Document 1); a method in which a water-absorbent resin is used obtained from heat treatment of a specific water-absorbent resin precursor with a specific surface crosslinking agent (see Patent Document 2); and the like.
However, these methods cannot necessarily satisfy an absorption performance expected for an absorbent material having a large amount of a water-absorbent resin. Further, they tend to cause a problem in that, for example, a to-be-absorbed liquid may not be captured, resulting in liquid leakage.